An explosion at a north London post office on Saturday night is believed to have been the work of the Real IRA. No one was injured in the attack, for which no warning was given.

The bombers who planted a device outside a Post Office delivery depot were labelled "totally reckless" on Sunday, as fears grew that a renewed mainland terror campaign would escalate.

The device, packed with up to 1lb of high explosives, blew out windows, exploding "without any notice whatsoever," said deputy assistant commissioner Alan Fry, head of Scotland Yard's anti-terrorist branch.

This explosion comes six weeks after a bomb blast hit the BBC TV Centre in London.

At the time Mr Fry said he feared it was the escalation of a Real IRA terror campaign on mainland Britain.

"We have been predicting, since Christmas, that the mainland, and London in particular, would be subject to terrorist attacks," he said.

Saturday's device exploded at 11.28pm and was a similar size to the device at Hammersmith Bridge in June last year.

It blew out the glass from the windows of the depot, but nobody was inside and there were no reports of injuries.

But it was designed to maim or injure, and falling glass from the blast could have caused horrendous injuries or even killed.

"Whoever put that device down was totally reckless as to someone living in the area, drinking in the pub just beyond this device, walking past at the time of the explosion or in a vehicle, could have had a serious accident," he said.

Lab tests will confirm if the bomb was the work of the Real IRA, he added.

"Due to the number of recent attacks and the nature of this one, I suspect that this is the work of the Real IRA," Mr Fry said.

No claim of responsibility had been made by Sunday night.

A forensic fingertip search of the site was carried out and the focus of the investigation switched to laboratory work to establish the type of explosive and device used.

It was still unclear why the bombers targeted the depot at The Hyde, Hendon, which suffered minor damage in the blast.

"I don't know of a significance other than being a Post Office premises. That will fit in with the Easter Uprising, which of course is being celebrated, the anniversary of that event is this weekend," Mr Fry said.

"There are a number of other features that might fit in. There's the Good Friday Agreement and where the peace process is, and equally there was the film of Michael Collins on the television last night.

"Whether any of these events featured in the planning, in the reasons behind this explosion, I don't know, but clearly they are all lines of inquiry and all possibilities."

The BBC bomb blast is also thought to be the work of the Real IRA and it detonated inside a taxi as bomb disposal experts attempted to carry out a controlled explosion.

One London Underground worker suffered minor injuries in the attack.

Before the blast at the BBC premises in Wood Lane, Shepherd's Bush, coded warnings had been received.

From the police cordon around 100 yards away, two smashed windows could be seen and metal railings lining the road outside the depot were twisted, apparently by the force of the blast.

Gerry Madden, delivery officer manager at the postal sorting office, said 80 people work at the depot but no one was on duty on Saturday night.

He said: "Half-a-dozen windows were broken, nothing at all inside. Everything inside is perfect apart from a few scorch marks on the wall.

"No staff were due to be here until Monday night when everything will carry on as normal."

Stephen Cunliffe, manager of the Glen pub, just 50 yards from the sorting office, said the pub hosted a late disco on Saturday night and it was still busy at 11.30pm when the explosion happened.

He said: "I could hardly hear the explosion because of the music. Police evacuated us at 11.45pm. I'm concerned that this could have happened, but I'm glad that no one was injured. It was just a stroke of luck."

Mr Fry said it was of the "utmost importance" those responsible were found and brought to justice.

The depot is near the busy Edgware Road, which reopened following the police search, and he appealed to anyone who had seen "strange incidents" in the area to come forward.

The post office building is expected to open on Tuesday morning after the Bank Holiday.

Anyone with information about the blast is asked to contact the confidential Anti-Terrorist Hotline on 0800 789 321.